1. Field of the Invention
The present invention relates to photopolymerized tackified acrylic terpolymers as pressure-sensitive adhesives and to tapes made therewith.
2. Description of the Related Art
Acrylate pressure-sensitive adhesives are well known in the art. In U.S. Pat. No. Re 24,906 (Ulrich), alkyl acrylate copolymers are described which are predominately alkyl esters of acrylic acid having from 4 to 14 carbon atoms, and further comprise a minor amount of a polar copolymerizable monomer such as acrylic acid. Such adhesives are widely popular as they are readily available and provide a good balance of tack, shear and peel properties at a relatively low cost. Methods for making these adhesives involve the use of large amounts of solvent which is an effective method for polymerization of acrylate adhesives, but one which is increasingly, subject to safety and environmental restrictions.
U.S. Pat. No. 4,418,120 (Kealy et al.) discloses a pressure-sensitive adhesive tape which is made by coating a backing sheet with a solution of isooctyl acrylate/acrylic acid copolymer containing a tackifying rosin ester and an antioxidant, evaporating the solvent, and crosslinking the adhesive. This process also involves the use of large amounts of solvent.
U.S. Pat. No. 4,500,683 (Hori et al.) discloses a pressure-sensitive adhesive composition containing as a polymer component, an addition-polymerization polymer of an acrylate based polymer having sticking properties at room temperature and one or more ethylenically unsaturated monomers capable of forming a homopolymer or copolymer having a glass transition point of at least 273.degree. K. The addition-polymerization polymer is prepared by polymerizing one or more ethylenically unsaturated monomers in the presence of the acryl-based polymer by solution polymerization or bulk polymerization using radical polymerization catalysts, but polymerization can be initiated by energy in the form of light, etc. It is disclosed that known additives such as "a coloring agent, fillers, an anti-aging agent, and a tackifier" may be used. No such additions are exemplified.
U.S. Pat. No. 3,888,247 (Stenvall) discloses a first aid bandage having an adhesive layer applied over a flexible, preferably microporous backing. The adhesive layer is an acrylate type pressure-sensitive copolymer adhesive as described in Ulrich, with a 94:6 ratio. The adhesive contains 40 percent of a highly stabilized ester resin tackifier, commercially available from Hercules Chemical Co. under the trade name Foral.TM. 85. U.S. Pat. No. 4,335,171 (Zenk) discloses a pressure-sensitive adhesive transfer tape in which the adhesive is a laminate of two physically dissimilar laminae. The first lamina is stated to be a soft pressure-sensitive adhesive consisting essentially of an isooctyl acrylate:acrylic acid copolymer wherein the acid present is in the range of 3-12% of the copolymer. The copolymer is tackified with 50 parts of a hydrogenated rosin ester or hydrogenated rosin acid.
U.S. Pat. No. 4,701,509, (Sun et al.) discloses a four-part acrylate, hot-melt, pressure-sensitive adhesive having a composition consisting of 2-ethyl hexyl acrylate, n-butyl acrylate, isobutyl methacrylate, and n-vinyl caprolactam for use on human skin. The adhesives are solution polymerized and are disclosed to have high cohesive strength. No tackifiers are disclosed for use with the adhesive.
U.S. Pat. No. 4,370,380 (Shah) discloses a hot-melt coated pressure-sensitive optically clear blend of 1-30% of a water-soluble polymer or copolymer of a vinyl lactam together with 70-99 percent of a tacky water-insoluble copolymer of an alkyl acrylate or methacrylate ester or mixture of esters and a copolymerizable monomer wherein the ester is substantially free from cross-linking. The blend can be made via mixing followed by solvent or vehicle volatilization. Polymers and copolymers of 1-vinyl-2-pyrrolidone are preferred. The use of tackifiers with these pressure-sensitive adhesives is neither disclosed or suggested. U.S. Pat. No. 4,510,197, (Shah) discloses a solvent-polymerized water-permeable pressure-sensitive adhesive comprising a modified copolymer of butyl acrylate containing N-vinyl-2-pyrrolidone and acrylic acid. The use of tackifiers is not disclosed.
U.S. Pat. No. 4,181,752 (Martens et al.) discloses a process for ultraviolet photopolymerization of alkyl acrylate esters and polar copolymerizable monomers to form the acrylate copolymer. Martens teaches that intensity and spectral distribution of the irradiation must be controlled in order to attain desirable cohesive strengths and peel resistance. The photopolymerization is preferably carried out in an inert atmosphere as oxygen tends to inhibit the reaction.
Additional patents disclose ultraviolet radiation of acrylate adhesives. U.S. Pat. No. 4,364,972 (Moon) discloses the use of N-vinylpyrrolidone as the polar copolymerizable monomer in the acrylate adhesive copolymer to provide a pressure-sensitive adhesive with high adhesion to automotive paints. U.S. Pat. No. 4,391,687 (Vesley) discloses the use of specific chromophore-substituted -halomethyl-s-triazines as photoactive crosslinkers for acrylate copolymers. U.S. Pat. No. 4,599,265 (Esmay) discloses a readily peelable pressure-sensitive adhesive tape, the adhesive layer of which is highly crosslinked and low in polar monomer content.
Benefits of ultraviolet-radiation polymerized adhesives include safety and environmental improvements due to the elimination of solvents in the processing. However, addition of tackifiers to such adhesives may be desired for certain applications. Such an addition typically results in processing difficulties. For example, the above-cited Moon patent, which concerns pressure-sensitive adhesives designed specifically to provide enhanced adhesion to automotive paints, teaches that tackifiers can be blended into the photoactive mixtures of monomers from which those pressure-sensitive adhesives are photopolymerized, but warns, at column 6, lines 3-12, that "the addition of any such material adds complexity and hence expense to an otherwise simple, straight forward, economical process, and is not preferred, except to achieve specific results". The Moon patent does not exemplify this teaching. However, the introduction of a tackifier into a photopolymerizable mixture of monomers often interferes with the photopolymerization and prevents the attainment of the desired adhesive and cohesive properties.
U.S. Pat. No. 4,243,500 (Glennon) discloses a pressure-sensitive adhesive formed from a composition comprising at least one monofunctional unsaturated acrylate ester monomer, essentially saturated tackifying resin polymers, non-crystallizing elastomeric material, and an initiator responsive to ultraviolet light, or other penetrating radiation. Glennon specifies the use of UV light within a wavelength range of 1800-4000 angstroms. The intensity of the lamps to which the adhesive is exposed is much higher than that of the lamps disclosed in Martens et al.
Glennon states that the tackifying resin may be a substance or mixture of substances selected from the group consisting of esters of rosin, hydrogenated esters of rosin, modified rosin esters, esters of polymerized rosin, esters of hydrogenated rosin, hydrocarbon resin, linear homopolymers of alpha-methyl styrene, alpha-pinene terpene hydrocarbon resin, aromatic modified C-5 hydrocarbon resin, vinyltoluene alpha-methyl styrene copolymer resins, beta-pinene terpene resins, polycyclic hydrocarbon resins, and technical hydroabietyl alcohol. However, many of these essentially saturated resin polymers (e.g., alphamethyl styrene materials) are unsuitable for use in the curing method of the above-cited Martens patent due to incompatibility, which results in phase-separation of the tackifying resin from the monomer mixture, excessive UV absorption which retards the photochemical reaction, and high reactivity with the monomers such that photopolymerization of the monomers is substantially prohibited. The Glennon adhesive requires the use of from about 50 parts to about 250 parts of tackifying resin per hundred parts acrylate ester monomer in order to obtain a satisfactory adhesion level. Such levels of tackifier increase the glass transition temperature and act as chain transfer agents during the polymerization. This results in the pressure-sensitive adhesives having lowered molecular weights and reduced cohesive strengths. Further, many tackifiers tend to migrate to the surface of the adhesive upon aging, deteriorating the adhesion.
U.S. Pat. No. 4,645,711, (Winslow et al.) discloses a removable pressure-sensitive adhesive tape, the adhesive layer of which is an emulsion polymerized copolymer of alkyl acrylate such as isooctyl acrylate and a small amount of emulsifier monomer and a tackifying resin selected from hydrogenated rosin esters, polyterpene, polymerized alkyl styrene, and polymerized petroleum derived monomer resins. Most of these resins are unsuitable for use in in situ photopolymerized acrylate pressure-sensitive adhesives.
Applicants have now discovered that from about 5 part to about 40 part of a rosin ester based tackifier will provide flexible pressure-sensitive adhesives which can be ultraviolet-radiation polymerized and which show a surprising shift in the dynamic mechanical properties such as glass transition temperature when used in acrylic terpolymer pressure-sensitive adhesives wherein the monomers include at least one alkyl acrylate and at least one strongly polar copolymerizable monomer. Monomers and tackifier may be selected such that adhesives with a wide range of tack, peel and shear properties are possible.